1. Field of the Invention
The present general inventive concept relates to an image forming apparatus. More particularly, the present general inventive concept relates to a power transmitting unit to transmit power between two members, and a developing device and an image forming apparatus having the same.
2. Description of the Related Art
Generally, conventional image forming apparatuses such as a printer, a copier, a facsimile machine, and a composite apparatus have all functions thereof have a printing function to print images on a printing medium. The image forming apparatuses have a printing medium feeding unit, a photosensitive medium, a developing device, a transferring unit, a fusing unit, and a conveying unit. Alternatively, a color image forming apparatus needs the developing device having a plurality of developing units for forming color images.
The color image forming apparatus has first to fourth developing units to store yellow, magenta, cyan, and black toners, respectively. The first to fourth developing devices have first to fourth developing members to face the photosensitive medium. Each of the first to fourth developing members has a gap ring disposed at one side thereof to maintain a gap between the photosensitive medium and the developing member. Each of the first to fourth developing units is biased toward the photosensitive medium by an elastic member disposed at a rear side of each of the developing units.
The first to fourth developing members receive a driving force from first to fourth driving members disposed in a main body of the image forming apparatus, and are driven. For this, first to fourth driven members engaging with the first to fourth driving members are coaxially disposed at the first to fourth developing members, respectively. Each of the first to fourth developing members rotates counterclockwise, and the photosensitive medium rotates clockwise.
The first to fourth developing units having the above-described structure are vertically arranged at one side of the photosensitive medium by predetermined intervals. At this time, the first to fourth developing units are guided along mounting rails disposed in the main body of the image forming apparatus, so mounted to the main body of the image forming apparatus so that each of the first to fourth driven members are engaged with each of the first to fourth driving members. Also, each of the first to fourth driven members has a mounting projection to be inserted into the mounting rail at one side thereof.
Alternatively, forces operating between the fourth driven member and the fourth driving member of the fourth developing unit that is disposed to face the photosensitive medium at a lower position compared to the first to third developing units are resolved as illustrated in FIG. 1.
Referring to FIG. 1, Fd is a driving force to operate between the fourth driven member 4 and the fourth driving member 3 engaging with each other, and Fb is an elastic pressing force to push the fourth developing unit toward the photosensitive medium 1 from a rear side of the fourth developing unit. Each of the driving force Fd and elastic pressing force Fb is resolved into a tangential force Fdt and Fbt and a normal force Fdc and Fbc between the fourth driven member 4 and the fourth driving member 3. A reference numeral 5 is a mounting rail.
The two normal forces Fdc and Fbc to determine a developing nip between the developing member 2 and the photosensitive medium 1 operate in opposite direction. The normal force Fdc caused by the driving force Fd is larger than the normal force Fbc caused by the elastic pressing force Fb. Therefore, the fourth driven member 4 moves in a direction of the Fdc. Because the fourth driven member 4 and fourth developing member 2 are disposed at the same shaft 2a, a developing nip between the photosensitive medium 1 and the fourth developing member 2 is changed so that developing quality is likely to degrade.
Alternatively, because an outer surface of the photosensitive medium 1 is formed in an ellipse section not a circle section, the fourth developing member 2 is required to move within a predetermined range for uniformly maintaining the developing nip between the photosensitive medium 1 and the developing member 2. If the movement of the fourth developing member 2 is transmitted to the fourth driven member 4 disposed at the same shaft 2a with the fourth developing member 2, the movement of the fourth developing member 2 impedes engagement between the fourth driving member 3 and the fourth driven member 4.
Also, the two tangential forces Fdt and Fbt resolved from the driving force Fd and the elastic pressing force Fb cause the fourth driven member 4 to move toward a bottom end of the photosensitive medium 1. As a result, a distance between a shaft of the fourth driven member 4 and a shaft of the fourth driving member 3 is unstable.